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Use Velocity and Displacement (Prox) Transducers with vb Instrument


QUESTION

Can I connect velocity or displacement transducers to my vbSeries portable instrument? What settings should I use on the instrument and what units will the displayed values use?

ANSWER 1 (vb6, vb7 and vb8 instruments)

vb6, vb7 and vb8 instruments can record both spectra and waveforms from velocity and displacement sensors. In the case of the displacement waveforms, they are not centred on 0 but are offset by the sensor gap reading (in displacement units). If desired, this offset can be removed by adjusting the sensor offset setting. The units of this reading are in displacement not voltage

Note: vb5 instrument models accept accelerometer sensors only.

To monitor and trend the sensor gap, a separate Average Value reading needs to be taken. Again the units of this reading are in displacement. Numeric Data Alarms can be applied to the reading. For a typical displacement probe with 2 mm (80 mil) range, appropriate limits may be 0.4 to 1.6 mm (15 - 65 mil).


ANSWER 2 (vb3000 model with serial numbers 11500 and higher)

vb3000 instruments that use firmware version 4.30 and higher can accept velocity and displacement transducers as well as current sensors. Only instruments with serial numbers 11500 and higher have the appropriate hardware to support this.

Note: The analogue inputs are still AC coupled, so it is not possible to read DC levels. For example you can read the vibration signal from prox probes but not their DC gap.

ANSWER 3 (Other vbX000 instruments)

Instruments running firmware versions prior to version 4.30 have accelerometer inputs only. However, this is not a hardware limitation; any suitable voltage signal can be fed into the vb inputs but the instrument firmware is not configured to scale the results into appropriate units.

It is possible to use other sensors provided you are only interested in the AC signal and you are prepared to let the instrument display the velocity or displacement signal incorrectly in units of gs.

Notes:

  • See FAQ Difficulty Measuring Low-Frequency, Large-Amplitude Displacement Signals to avoid problems when measuring low frequency, large amplitude signals.
  • The inputs are AC coupled, so frequencies below 1 Hz are attenuated (see product specification sheet). This is significant for displacement probes, for which the DC position often contains important information.

How to use a Sensor other than an Accelerometer (vbX000):

  • In the Sensor Setup Menu define a new sensor e.g. 'Velocity Trans'.
  • Enter a Sensitivity e.g. 100 mV/g in place of 100 mV/in/s for velocity transducers or 200 mV/g instead of 200 mV/mil for displacement transducers (the instrument supports sensitivities between 8.5 and 2300).
  • Set the Drive Current ON for ICP® sensors (with 2 mA supplied) and OFF for other input types.
  • Use the instrument in acceleration mode, not velocity or displacement.
  • Use units of gs. The correct values will be displayed but in units of g instead of in/s (or mil).
  • To present a report to a customer, paste it into Word® and manually edit or obscure the units.


This work-around may not be suitable for all applications where velocity or displacement transducers would normally be used. However, this may be acceptable for in-house use, or for occasional use of sensors other than accelerometers.

MORE INFORMATION

Read the FAQ article Current Readings with the vb Instrument.